To Love In The 57th Century (sengen)
13 parts Ongoing Senku and Gen were childhood friends, though they seemed to feel more. The two were inseparable until Gen moved away because of his father's job promotion. Ever since then, they sent letters to each other, each year becoming less and less as they grew apart.
Until Gen transferred into Senku's highschool, then got placed into Senku's class, and got grouped for a cooperative work. They now can't avoid to face their feelings and their past.